Supraventricular tachyarrhythmia discrimination

ABSTRACT

Techniques are described for discriminating SVT and, in particular, rapidly conducting AF. The techniques include detecting an onset of a fast rate of ventricular events sensed from a cardiac electrical signal and detecting a pause in the fast rate of ventricular sensed events. A threshold number of ventricular event intervals required to detect a ventricular tachyarrhythmia is detected with each of the threshold number of ventricular event intervals being less than a tachyarrhythmia detection interval. Detection of the ventricular tachyarrhythmia and an electrical stimulation therapy for treating the ventricular tachyarrhythmia are withheld in response to at least the pause being detected.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.16/217,207, filed on Dec. 12, 2018, which claims the benefit of thefiling date of provisional U.S. Patent Application No. 62/599,071, filedDec. 15, 2017, both of which are incorporated herein by reference intheir entirety.

TECHNICAL FIELD

The disclosure relates generally to a medical device system and methodfor discriminating supraventricular tachyarrhythmia, particularlyrapidly conductive atrial fibrillation, from ventriculartachyarrhythmia.

BACKGROUND

Medical devices, such as cardiac pacemakers and implantable cardioverterdefibrillators (ICDs), provide therapeutic electrical stimulation to aheart of a patient via electrodes carried by one or more medicalelectrical leads and/or electrodes on a housing of the medical device.The electrical stimulation may include signals such as pacing pulses orcardioversion or defibrillation shocks. In some cases, a medical devicemay sense cardiac electrical signals attendant to the intrinsic orpacing-evoked depolarizations of the heart and control delivery ofstimulation signals to the heart based on sensed cardiac electricalsignals.

Upon detection of an abnormal rhythm, such as bradycardia, tachycardiaor fibrillation, an appropriate electrical stimulation signal or signalsmay be delivered to restore or maintain a more normal rhythm of theheart. For example, an ICD may deliver pacing pulses to the heart of thepatient upon detecting bradycardia or tachycardia or delivercardioversion or defibrillation shocks to the heart upon detectingtachycardia or fibrillation. The ICD may sense the cardiac electricalsignals in a heart chamber and deliver electrical stimulation therapiesto the heart chamber using electrodes carried by transvenous medicalelectrical leads. Cardiac signals sensed within the heart generally havea high signal strength and quality for reliably sensing cardiacelectrical events, such as R-waves associated with ventricular events.In other examples, a non-transvenous lead may be coupled to the ICD, inwhich case cardiac signal sensing presents new challenges in accuratelysensing cardiac electrical events and properly detecting anddiscriminating between different types of cardiac arrhythmias.

Proper detection and discrimination of different tachyarrhythmias isimportant in automatically selecting and delivering an effectiveelectrical stimulation therapy by an implantable medical device systemand avoiding unnecessary therapies. For example, a supraventriculartachyarrhythmia originates in the upper, atrial heart chambers and isconducted to the lower, ventricular heart chambers. A supraventriculartachyarrhythmia (SVT) is generally not successfully terminated bydelivering electrical stimulation therapy to the ventricles because theheart rhythm is arising from the upper heart chambers. A ventriculartachyarrhythmia that originates in the lower, ventricular heartchambers, on the other hand, generally can be successfully treated bydelivering electrical stimulation therapies to the ventricles toterminate the abnormal ventricular rhythm. Accordingly, discriminationof supraventricular tachyarrhythmia that originates in the upper heartchambers from ventricular tachyarrhythmia that originates in the lowerheart chambers allows for appropriate therapy selection and deliverywhile avoiding unnecessary or potentially ineffective electricalstimulation therapy from being delivered to the patient's heart.

SUMMARY

In general, the disclosure is directed to techniques for discriminatingSVT from ventricular tachyarrhythmias, e.g., ventricular tachycardia(VT) and ventricular fibrillation (VF), and withholding VT and VFdetection and therapies when SVT is detected. In some examples, amedical device system, such as an ICD system, operating according to thetechniques disclosed herein may detect rapidly conducted atrialfibrillation (AF) by detecting a pause in the rate of sensed ventricularevents. The pause in the rate of sensed ventricular events may bedetected based on at least one relatively long interval betweenconsecutively sensed R-waves and morphology features of a cardiacelectrical signal. If a pause is detected in a fast ventricular rate isdetected and other VT or VF detection criteria are satisfied, the VT orVF detection and therapy may be delayed or withheld.

In one example, the disclosure provides a device comprising a therapydelivery circuit configured to generate an electrical stimulationtherapy, a sensing circuit configured to receive a first cardiacelectrical signal via a first sensing electrode vector and senseventricular events from the first cardiac electrical signal, and acontrol circuit coupled to the sensing circuit and the therapy deliverycircuit. The control circuit is configured to determine that a firstplurality of the sensed ventricular events meet a fast ventricular ratecriteria; subsequent to determining the fast ventricular rate criteriais met, detect a pause in a rate of a second plurality of the sensedventricular events; detect from the first cardiac electrical signal athreshold number of ventricular event intervals required to detect aventricular tachyarrhythmia, each of the threshold number of ventricularevent intervals being less than a tachyarrhythmia detection interval;and withhold the electrical stimulation therapy for treating theventricular tachyarrhythmia in response to at least the pause beingdetected.

In another example, the disclosure provides a method comprisingreceiving a first cardiac electrical signal; sensing ventricular eventsfrom the first cardiac electrical signal; determining that a firstplurality of the sensed ventricular events meet a fast ventricular ratecriteria; subsequent to determining the fast ventricular rate criteriais met, detecting a pause in a rate of a second plurality of the sensedventricular events; detecting from the first cardiac electrical signal athreshold number of ventricular event intervals required to detect aventricular tachyarrhythmia, each of the threshold number of ventricularevent intervals being less than a tachyarrhythmia detection interval;and withholding an electrical stimulation therapy for treating theventricular tachyarrhythmia in response to at least the pause beingdetected.

In another example, the disclosure provides a non-transitory,computer-readable storage medium comprising a set of instructions which,when executed by a processor, cause the processor to receive a firstcardiac electrical signal; sense ventricular events from the firstcardiac electrical signal; determine that a first plurality of thesensed ventricular events meet a fast ventricular rate criteria;subsequent to determining the fast ventricular rate criteria is met,detect a pause in a rate of a second plurality of the sensed ventricularevents; detect from the first cardiac electrical signal a thresholdnumber of ventricular event intervals required to detect a ventriculartachyarrhythmia, each of the threshold number of ventricular eventintervals being less than a tachyarrhythmia detection interval; andwithhold an electrical stimulation therapy for treating the ventriculartachyarrhythmia in response to at least the pause being detected.

This summary is intended to provide an overview of the subject matterdescribed in this disclosure. It is not intended to provide an exclusiveor exhaustive explanation of the apparatus and methods described indetail within the accompanying drawings and description below. Furtherdetails of one or more examples are set forth in the accompanyingdrawings and the description below.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A and 1B are conceptual diagrams of an extra-cardiovascular ICDsystem according to one example.

FIGS. 2A-2C are conceptual diagrams of a patient implanted with theextra-cardiovascular ICD system of FIG. 1A in a different implantconfiguration.

FIG. 3 is a schematic diagram of the ICD of FIGS. 1A-2C according to oneexample.

FIG. 4 is diagram of circuitry included in the sensing circuit of FIG. 3according to one example.

FIG. 5 is a flow chart of a method performed by an ICD for detecting apause in a rapidly conducted AF rhythm.

FIG. 6 is a conceptual diagram of a pause in a conducted AF rhythm thatmay be detected by an ICD.

FIG. 7 is a flow chart of a method for detecting ventriculartachyarrhythmias according to one example using the rapidly conducted AFdetection techniques disclosed herein.

DETAILED DESCRIPTION

In general, this disclosure describes techniques for discriminating SVTfrom VT and VF by a cardiac medical device or system and withholdingdetection of a ventricular tachyarrhythmia or therapy to treat theventricular tachyarrhythmia in response to detecting SVT. Criteria fordetecting ventricular tachyarrhythmia, such as heart rate-basedcriteria, may become satisfied in the presence of SVT. As such, heartrate alone may be insufficient for reliably discriminating between SVTand VT/VF. Techniques for detecting SVT as described herein allow atachyarrhythmia detection and/or therapy to be withheld or delayed whenevidence of SVT is identified.

In some examples, the cardiac medical device system may be anextra-cardiovascular ICD system. As used herein, the term“extra-cardiovascular” refers to a position outside the blood vessels,heart, and pericardium surrounding the heart of a patient. Implantableelectrodes carried by extra-cardiovascular leads may be positionedextra-thoracically (outside the ribcage and sternum) orintra-thoracically (beneath the ribcage or sternum) but generally not inintimate contact with myocardial tissue. The techniques disclosed hereinfor detecting SVT and withholding a VT/VF detection may be applied to acardiac electrical signal acquired using extra-cardiovascularelectrodes.

These techniques are presented herein in conjunction with an ICD andimplantable medical lead carrying extra-cardiovascular electrodes, butaspects of the techniques may be utilized in conjunction with othercardiac medical devices or systems. For example, the techniques fordetecting SVT, such as rapidly conductive atrial fibrillation, asdescribed in conjunction with the accompanying drawings may beimplemented in any implantable or external medical device enabled forsensing cardiac electrical signals, including implantable pacemakers,ICDs or cardiac monitors coupled to transvenous, pericardial orepicardial leads carrying sensing and therapy delivery electrodes;leadless pacemakers, ICDs or cardiac monitors having housing-basedsensing electrodes; and external or wearable pacemakers, defibrillators,or cardiac monitors coupled to external, surface or skin electrodes.

FIGS. 1A and 1B are conceptual diagrams of an extra-cardiovascular ICDsystem 10 according to one example. FIG. 1A is a front view of ICDsystem 10 implanted within patient 12. FIG. 1B is a side view of ICDsystem 10 implanted within patient 12. ICD system 10 includes an ICD 14connected to an extra-cardiovascular electrical stimulation and sensinglead 16. FIGS. 1A and 1B are described in the context of an ICD system10 capable of providing defibrillation and/or cardioversion shocks andpacing pulses.

ICD 14 includes a housing 15 that forms a hermetic seal that protectsinternal components of ICD 14. The housing 15 of ICD 14 may be formed ofa conductive material, such as titanium or titanium alloy. The housing15 may function as an electrode (sometimes referred to as a “can”electrode). Housing 15 may be used as an active can electrode for use indelivering cardioversion/defibrillation (CV/DF) shocks or other highvoltage pulses delivered using a high voltage therapy circuit. In otherexamples, housing 15 may be available for use in delivering unipolar,low voltage cardiac pacing pulses and/or for sensing cardiac electricalsignals in combination with electrodes carried by lead 16. In otherinstances, the housing 15 of ICD 14 may include a plurality ofelectrodes on an outer portion of the housing. The outer portion(s) ofthe housing 15 functioning as an electrode(s) may be coated with amaterial, such as titanium nitride, e.g., for reducing post-stimulationpolarization artifact.

ICD 14 includes a connector assembly 17 (also referred to as a connectorblock or header) that includes electrical feedthroughs crossing housing15 to provide electrical connections between conductors extending withinthe lead body 18 of lead 16 and electronic components included withinthe housing 15 of ICD 14. As will be described in further detail herein,housing 15 may house one or more processors, memories, transceivers,electrical cardiac signal sensing circuitry, therapy delivery circuitry,power sources and other components for sensing cardiac electricalsignals, detecting a heart rhythm, and controlling and deliveringelectrical stimulation pulses to treat an abnormal heart rhythm.

Elongated lead body 18 has a proximal end 27 that includes a leadconnector (not shown) configured to be connected to ICD connectorassembly 17 and a distal portion 25 that includes one or moreelectrodes. In the example illustrated in FIGS. 1A and 1B, the distalportion 25 of lead body 18 includes defibrillation electrodes 24 and 26and pace/sense electrodes 28 and 30. In some cases, defibrillationelectrodes 24 and 26 may together form a defibrillation electrode inthat they may be configured to be activated concurrently. Alternatively,defibrillation electrodes 24 and 26 may form separate defibrillationelectrodes in which case each of the electrodes 24 and 26 may beactivated independently.

Electrodes 24 and 26 (and in some examples housing 15) are referred toherein as defibrillation electrodes because they are utilized,individually or collectively, for delivering high voltage stimulationtherapy (e.g., cardioversion or defibrillation shocks). Electrodes 24and 26 may be elongated coil electrodes and generally have a relativelyhigh surface area for delivering high voltage electrical stimulationpulses compared to pacing and sensing electrodes 28 and 30. However,electrodes 24 and 26 and housing 15 may also be utilized to providepacing functionality, sensing functionality or both pacing and sensingfunctionality in addition to or instead of high voltage stimulationtherapy. In this sense, the use of the term “defibrillation electrode”herein should not be considered as limiting the electrodes 24 and 26 foruse in only high voltage cardioversion/defibrillation shock therapyapplications. For example, electrodes 24 and 26 may be used in a sensingvector used to sense cardiac electrical signals and detect anddiscriminate SVT, VT and VF.

Electrodes 28 and 30 are relatively smaller surface area electrodeswhich are available for use in sensing electrode vectors for sensingcardiac electrical signals and may be used for delivering relatively lowvoltage pacing pulses in some configurations. Electrodes 28 and 30 arereferred to as pace/sense electrodes because they are generallyconfigured for use in low voltage applications, e.g., used as either acathode or anode for delivery of pacing pulses and/or sensing of cardiacelectrical signals, as opposed to delivering high voltage cardioversiondefibrillation shocks. In some instances, electrodes 28 and 30 mayprovide only pacing functionality, only sensing functionality or both.

ICD 14 may obtain cardiac electrical signals corresponding to electricalactivity of heart 8 via a combination of sensing vectors that includecombinations of electrodes 24, 26, 28 and/or 30. In some examples,housing 15 of ICD 14 is used in combination with one or more ofelectrodes 24, 26, 28 and/or 30 in a sensing electrode vector. Varioussensing electrode vectors utilizing combinations of electrodes 24, 26,28, and 30 and housing 15 are described below for acquiring first andsecond cardiac electrical signals using respective first and/or secondsensing electrode vectors that may be selected by sensing circuitryincluded in ICD 14.

In the example illustrated in FIGS. 1A and 1B, electrode 28 is locatedproximal to defibrillation electrode 24, and electrode 30 is locatedbetween defibrillation electrodes 24 and 26. One, two or more pace/senseelectrodes may be carried by lead body 18. For instance, a thirdpace/sense electrode may be located distal to defibrillation electrode26 in some examples. Electrodes 28 and 30 are illustrated as ringelectrodes; however, electrodes 28 and 30 may comprise any of a numberof different types of electrodes, including ring electrodes, short coilelectrodes, hemispherical electrodes, directional electrodes, segmentedelectrodes, or the like. Electrodes 28 and 30 may be positioned at anylocation along lead body 18 and are not limited to the positions shown.In other examples, lead 16 may include none, one or more pace/senseelectrodes and/or one or more defibrillation electrodes.

In the example shown, lead 16 extends subcutaneously or submuscularlyover the ribcage 32 medially from the connector assembly 27 of ICD 14toward a center of the torso of patient 12, e.g., toward xiphoid process20 of patient 12. At a location near xiphoid process 20, lead 16 bendsor turns and extends superior subcutaneously or submuscularly over theribcage and/or sternum, substantially parallel to sternum 22. Althoughillustrated in FIGS. 1A as being offset laterally from and extendingsubstantially parallel to sternum 22, the distal portion 25 of lead 16may be implanted at other locations, such as over sternum 22, offset tothe right or left of sternum 22, angled laterally from sternum 22 towardthe left or the right, or the like. Alternatively, lead 16 may be placedalong other subcutaneous or submuscular paths. The path ofextra-cardiovascular lead 16 may depend on the location of ICD 14, thearrangement and position of electrodes carried by the lead body 18,and/or other factors.

Electrical conductors (not illustrated) extend through one or morelumens of the elongated lead body 18 of lead 16 from the lead connectorat the proximal lead end 27 to electrodes 24, 26, 28, and 30 locatedalong the distal portion 25 of the lead body 18. The elongatedelectrical conductors contained within the lead body 18 are eachelectrically coupled with respective defibrillation electrodes 24 and 26and pace/sense electrodes 28 and 30, which may be separate respectiveinsulated conductors within the lead body 18. The respective conductorselectrically couple the electrodes 24, 26, 28, and 30 to circuitry, suchas a therapy delivery circuit and/or a sensing circuit, of ICD 14 viaconnections in the connector assembly 17, including associatedelectrical feedthroughs crossing housing 15. The electrical conductorstransmit therapy from a therapy delivery circuit within ICD 14 to one ormore of defibrillation electrodes 24 and 26 and/or pace/sense electrodes28 and 30 and transmit sensed electrical signals from one or more ofdefibrillation electrodes 24 and 26 and/or pace/sense electrodes 28 and30 to the sensing circuit within ICD 14.

The lead body 18 of lead 16 may be formed from a non-conductivematerial, including silicone, polyurethane, fluoropolymers, mixturesthereof, and other appropriate materials, and shaped to form one or morelumens within which the one or more conductors extend. Lead body 18 maybe tubular or cylindrical in shape. In other examples, the distalportion 25 (or all of) the elongated lead body 18 may have a flat,ribbon or paddle shape. Lead body 18 may be formed having a preformeddistal portion 25 that is generally straight, curving, bending,serpentine, undulating or zig-zagging.

In the example shown, lead body 18 includes a curving distal portion 25having two “C” shaped curves, which together may resemble the Greekletter epsilon, “ε.” Defibrillation electrodes 24 and 26 are eachcarried by one of the two respective C-shaped portions of the lead bodydistal portion 25. The two C-shaped curves are seen to extend or curvein the same direction away from a central axis of lead body 18, alongwhich pace/sense electrodes 28 and 30 are positioned. Pace/senseelectrodes 28 and 30 may, in some instances, be approximately alignedwith the central axis of the straight, proximal portion of lead body 18such that mid-points of defibrillation electrodes 24 and 26 arelaterally offset from pace/sense electrodes 28 and 30.

Other examples of extra-cardiovascular leads including one or moredefibrillation electrodes and one or more pacing and sensing electrodescarried by curving, serpentine, undulating or zig-zagging distal portionof the lead body 18 that may be implemented with the techniquesdescribed herein are generally disclosed in pending U.S. Pat.Publication No. 2016/0158567 (Marshall, et al.), incorporated herein byreference in its entirety. The techniques disclosed herein are notlimited to any particular lead body design, however. In other examples,lead body 18 is a flexible elongated lead body without any pre-formedshape, bends or curves. Various example configurations ofextra-cardiovascular leads and electrodes and dimensions that may beimplemented in conjunction with the SVT discrimination techniquesdisclosed herein are described in pending U.S. Publication No.2015/0306375 (Marshall, et al.) and pending U.S. Publication No.2015/0306410 (Marshall, et al.), both of which are incorporated hereinby reference in their entirety.

ICD 14 analyzes the cardiac electrical signals received from one or moresensing electrode vectors to monitor for abnormal rhythms, such asbradycardia, SVT, VT or VF. ICD 14 may analyze the heart rate andmorphology of the cardiac electrical signals to monitor fortachyarrhythmia in accordance with any of a number of tachyarrhythmiadetection techniques. One example technique for detectingtachyarrhythmia is described in U.S. Pat. No. 7,761,150 (Ghanem, etal.), incorporated herein by reference in its entirety. Exampletechniques for detecting VT and VF are described below in conjunctionwith the accompanying figures. The techniques for discriminating SVTfrom VT or VF for withholding a VT or VF detection as disclosed hereinmay be incorporated in a variety of VT/VF detection algorithms. Examplesof devices and tachyarrhythmia detection algorithms that may be adaptedto utilize techniques for SVT discrimination described herein aregenerally disclosed in U.S. Pat. No. 5,354,316 (Keimel); U.S. Pat. No.5,545,186 (Olson, et al.); U.S. Pat. No. 6,393,316 (Gillberg et al.);U.S. Pat. No. 7,031,771 (Brown, et al.); U.S. Pat. No. 8,160,684(Ghanem, et al.), and U.S. Pat. No. 8,437,842 (Zhang, et al.), all ofwhich patents are incorporated herein by reference in their entirety.

ICD 14 generates and delivers electrical stimulation therapy in responseto detecting a tachyarrhythmia (e.g., VT or VF) using a therapy deliveryelectrode vector which may be selected from any of the availableelectrodes 24, 26, 28 30 and/or housing 15. ICD 14 may deliver ATP inresponse to VT detection, and in some cases may deliver ATP prior to aCV/DF shock or during high voltage capacitor charging in an attempt toavert the need for delivering a CV/DF shock. If ATP does notsuccessfully terminate VT or when VF is detected, ICD 14 may deliver oneor more CV/DF shocks via one or both of defibrillation electrodes 24 and26 and/or housing 15. ICD 14 may deliver the CV/DF shocks usingelectrodes 24 and 26 individually or together as a cathode (or anode)and with the housing 15 as an anode (or cathode). ICD 14 may generateand deliver other types of electrical stimulation pulses such aspost-shock pacing pulses or bradycardia pacing pulses using a pacingelectrode vector that includes one or more of the electrodes 24, 26, 28,and 30 and the housing 15 of ICD 14.

FIGS. 1A and 1B are illustrative in nature and should not be consideredlimiting of the practice of the techniques disclosed herein. ICD 14 isshown implanted subcutaneously on the left side of patient 12 along theribcage 32. ICD 14 may, in some instances, be implanted between the leftposterior axillary line and the left anterior axillary line of patient12. ICD 14 may, however, be implanted at other subcutaneous orsubmuscular locations in patient 12. For example, ICD 14 may beimplanted in a subcutaneous pocket in the pectoral region. In this case,lead 16 may extend subcutaneously or submuscularly from ICD 14 towardthe manubrium of sternum 22 and bend or turn and extend inferiorly fromthe manubrium to the desired location subcutaneously or submuscularly.In yet another example, ICD 14 may be placed abdominally. Lead 16 may beimplanted in other extra-cardiovascular locations as well. For instance,as described with respect to FIGS. 2A-2C, the distal portion 25 of lead16 may be implanted underneath the sternum/ribcage in the substernalspace.

An external device 40 is shown in telemetric communication with ICD 14by a communication link 42. External device 40 may include a processor,display, user interface, telemetry unit and other components forcommunicating with ICD 14 for transmitting and receiving data viacommunication link 42. Communication link 42 may be established betweenICD 14 and external device 40 using a radio frequency (RF) link such asBLUETOOTH® communication, Wi-Fi, or Medical Implant CommunicationService (MICS) or other RF or communication frequency bandwidth.

External device 40 may be embodied as a programmer used in a hospital,clinic or physician's office to retrieve data from ICD 14 and to programoperating parameters and algorithms in ICD 14 for controlling ICDfunctions. External device 40 may be used to program cardiac eventsensing parameters (e.g., R-wave sensing parameters), cardiac rhythmdetection parameters (e.g., VT and VF detection parameters and SVTdiscrimination parameters) and therapy control parameters used by ICD14. Data stored or acquired by ICD 14, including physiological signalsor associated data derived therefrom, results of device diagnostics, andhistories of detected rhythm episodes and delivered therapies, may beretrieved from ICD 14 by external device 40 following an interrogationcommand. External device 40 may alternatively be embodied as a homemonitor or hand held device.

FIGS. 2A-2C are conceptual diagrams of patient 12 implanted withextra-cardiovascular ICD system 10 in a different implant configurationthan the arrangement shown in FIGS. 1A-1B. FIG. 2A is a front view ofpatient 12 implanted with ICD system 10. FIG. 2B is a side view ofpatient 12 implanted with ICD system 10. FIG. 2C is a transverse view ofpatient 12 implanted with ICD system 10. In this arrangement,extra-cardiovascular lead 16 of system 10 is implanted at leastpartially underneath sternum 22 of patient 12. Lead 16 extendssubcutaneously or submuscularly from ICD 14 toward xiphoid process 20and at a location near xiphoid process 20 bends or turns and extendssuperiorly within anterior mediastinum 36 in a substernal position.

Anterior mediastinum 36 may be viewed as being bounded laterally bypleurae 39, posteriorly by pericardium 38, and anteriorly by sternum 22(see FIG. 2C). The distal portion 25 of lead 16 may extend along theposterior side of sternum 22 substantially within the loose connectivetissue and/or substernal musculature of anterior mediastinum 36. A leadimplanted such that the distal portion 25 is substantially withinanterior mediastinum 36, may be referred to as a “substernal lead.”

In the example illustrated in FIGS. 2A-2C, lead 16 is locatedsubstantially centered under sternum 22. In other instances, however,lead 16 may be implanted such that it is offset laterally from thecenter of sternum 22. In some instances, lead 16 may extend laterallysuch that distal portion 25 of lead 16 is underneath/below the ribcage32 in addition to or instead of sternum 22. In other examples, thedistal portion 25 of lead 16 may be implanted in otherextra-cardiovascular, intra-thoracic locations, including the pleuralcavity or around the perimeter of and adjacent to but typically notwithin the pericardium 38 of heart 8. Other implant locations and leadand electrode arrangements that may be used in conjunction with the SVTdiscrimination techniques described herein are generally disclosed inthe above-incorporated references.

FIG. 3 is a schematic diagram of ICD 14 according to one example. Theelectronic circuitry enclosed within housing 15 (shown schematically asan electrode in FIG. 3) includes software, firmware and hardware thatcooperatively monitor cardiac electrical signals, determine when anelectrical stimulation therapy is necessary, and deliver therapies asneeded according to programmed therapy delivery algorithms and controlparameters. The software, firmware and hardware are configured to detecttachyarrhythmias and deliver anti-tachyarrhythmia therapy, e.g., detectventricular tachyarrhythmias and in some cases discriminate VT from VFfor determining when ATP or CV/DF shocks are required. ICD 14 is coupledto an extra-cardiovascular lead, such as lead 16 carryingextra-cardiovascular electrodes 24, 26, 28, and 30, for deliveringelectrical stimulation pulses to the patient's heart and for sensingcardiac electrical signals.

ICD 14 includes a control circuit 80, memory 82, therapy deliverycircuit 84, sensing circuit 86, and telemetry circuit 88. A power source98 provides power to the circuitry of ICD 14, including each of thecomponents 80, 82, 84, 86, and 88 as needed. Power source 98 may includeone or more energy storage devices, such as one or more rechargeable ornon-rechargeable batteries. The connections between power source 98 andeach of the other components 80, 82, 84, 86 and 88 are to be understoodfrom the general block diagram of FIG. 3, but are not shown for the sakeof clarity. For example, power source 98 may be coupled to one or morecharging circuits included in therapy delivery circuit 84 for chargingholding capacitors included in therapy delivery circuit 84 that aredischarged at appropriate times under the control of control circuit 80for producing electrical pulses according to a therapy protocol, such asfor bradycardia pacing, post-shock pacing, ATP and/or CV/DF shockpulses. Power source 98 is also coupled to components of sensing circuit86, such as sense amplifiers, analog-to-digital converters, switchingcircuitry, etc. as needed.

The functional blocks shown in FIG. 3 represent functionality includedin ICD 14 and may include any discrete and/or integrated electroniccircuit components that implement analog and/or digital circuits capableof producing the functions attributed to ICD 14 herein. The variouscomponents may include an application specific integrated circuit(ASIC), an electronic circuit, a processor (shared, dedicated, or group)and memory that execute one or more software or firmware programs, acombinational logic circuit, state machine, or other suitable componentsor combinations of components that provide the described functionality.The particular form of software, hardware and/or firmware employed toimplement the functionality disclosed herein will be determinedprimarily by the particular system architecture employed in the ICD andby the particular detection and therapy delivery methodologies employedby the ICD. Providing software, hardware, and/or firmware to accomplishthe described functionality in the context of any modern ICD system,given the disclosure herein, is within the abilities of one of skill inthe art.

Memory 82 may include any volatile, non-volatile, magnetic, orelectrical non-transitory computer readable storage media, such asrandom access memory (RAM), read-only memory (ROM), non-volatile RAM(NVRAM), electrically-erasable programmable ROM (EEPROM), flash memory,or any other memory device. Furthermore, memory 82 may includenon-transitory computer readable media storing instructions that, whenexecuted by one or more processing circuits, cause control circuit 80and/or other ICD components to perform various functions attributed toICD 14 or those ICD components. The non-transitory computer-readablemedia storing the instructions may include any of the media listedabove.

The functions attributed to ICD 14 herein may be embodied as one or moreintegrated circuits. Depiction of different features as circuits isintended to highlight different functional aspects and does notnecessarily imply that such circuits must be realized by separatehardware or software components. Rather, functionality associated withone or more circuits may be performed by separate hardware, firmware orsoftware components, or integrated within common hardware, firmware orsoftware components. For example, cardiac event sensing andtachyarrhythmia detection operations may be performed cooperatively bysensing circuit 86 and control circuit 80 and may include operationsimplemented in a processor or other signal processing circuitry includedin control circuit 80 executing instructions stored in memory 82 andcontrol signals such as blanking and timing intervals and sensingthreshold amplitude signals sent from control circuit 80 to sensingcircuit 86.

Control circuit 80 communicates, e.g., via a data bus, with therapydelivery circuit 84 and sensing circuit 86 for sensing cardiacelectrical activity, detecting cardiac rhythms, and controlling deliveryof cardiac electrical stimulation therapies in response to sensedcardiac signals. Therapy delivery circuit 84 and sensing circuit 86 areelectrically coupled to electrodes 24, 26, 28, 30 carried by lead 16 andthe housing 15, which may function as a common or ground electrode or asan active can electrode for delivering CV/DF shock pulses or cardiacpacing pulses.

Sensing circuit 86 may be selectively coupled to electrodes 28, 30and/or housing 15 in order to monitor electrical activity of thepatient's heart. Sensing circuit 86 may additionally be selectivelycoupled to defibrillation electrodes 24 and/or 26 for use in a sensingelectrode vector together or in combination with one or more ofelectrodes 28, 30 and/or housing 15. Sensing circuit 86 may be enabledto selectively receive cardiac electrical signals from at least twosensing electrode vectors from the available electrodes 24, 26, 28, 30,and housing 15. At least two cardiac electrical signals from twodifferent sensing electrode vectors may be received simultaneously bysensing circuit 86. Sensing circuit 86 may monitor one or both or thecardiac electrical signals at a time for sensing cardiac electricalevents, e.g., P-waves attendant to the depolarization of the atrialmyocardium and/or R-waves attendant to the depolarization of theventricular myocardium, and providing digitized cardiac signal waveformsfor analysis by control circuit 80. For example, sensing circuit 86 mayinclude switching circuitry (not shown) for selecting which ofelectrodes 24, 26, 28, 30, and housing 15 are coupled to a first sensingchannel 83 and which are coupled to a second sensing channel 85 ofsensing circuit 86. Switching circuitry may include a switch array,switch matrix, multiplexer, or any other type of switching devicesuitable to selectively couple components of sensing circuit 86 toselected electrodes.

Each sensing channel 83 and 85 may be configured to amplify, filter anddigitize the cardiac electrical signal received from selected electrodescoupled to the respective sensing channel to improve the signal qualityfor detecting cardiac electrical events, such as R-waves or performingother signal analysis. The cardiac event detection circuitry withinsensing circuit 86 may include one or more sense amplifiers, filters,rectifiers, threshold detectors, comparators, analog-to-digitalconverters (ADCs), timers or other analog or digital components asdescribed further in conjunction with FIG. 4. A cardiac event sensingthreshold may be automatically adjusted by sensing circuit 86 under thecontrol of control circuit 80, based on timing intervals and sensingthreshold values determined by control circuit 80, stored in memory 82,and/or controlled by hardware, firmware and/or software of controlcircuit 80 and/or sensing circuit 86.

Upon detecting a cardiac electrical event based on a sensing thresholdcrossing, sensing circuit 86 may produce a sensed event signal, such asan R-wave sensed event signal, that is passed to control circuit 80. Insome examples, the sensed event signal may be used by control circuit 80to trigger storage of a segment of a cardiac electrical signal foranalysis for confirming the R-wave sensed event signals anddiscriminating SVT as described below.

The R-wave sensed event signals are also used by control circuit 80 fordetermining ventricular event intervals, referred to herein as “RRintervals” or “RRIs” for detecting tachyarrhythmia and determining aneed for therapy. A ventricular event interval or RRI is the timeinterval between two consecutively sensed R-waves and may be determinedbetween two consecutive R-wave sensed event signals received fromsensing circuit 86. In other words, a ventricular even interval or RRIis the time interval between a first R-wave and a second R-wave theimmediately follows the first R-wave. For example, control circuit 80may include a timing circuit 90 for determining RRIs between consecutiveR-wave sensed event signals received from sensing circuit 86 and forcontrolling various timers and/or counters used to control the timing oftherapy delivery by therapy delivery circuit 84. Timing circuit 90 mayadditionally set time windows such as morphology template windows,morphology analysis windows or perform other timing related functions ofICD 14 including synchronizing CV/DF shocks or other therapies deliveredby therapy delivery circuit 84 with sensed cardiac events.

Tachyarrhythmia detector 92 is configured to analyze signals receivedfrom sensing circuit 86 for detecting tachyarrhythmia episodes.Tachyarrhythmia detector 92 may be implemented in control circuit 80 assoftware, hardware and/or firmware that processes and analyzes signalsreceived from sensing circuit 86 for detecting VT and/or VF. In someexamples, tachyarrhythmia detector 92 may include comparators andcounters for counting RRIs determined by timing circuit 90 that fallinto various rate detection zones for determining a ventricular rate orperforming other rate- or interval-based assessments for detecting anddiscriminating VT and VF. For example, tachyarrhythmia detector 92 maycompare the RRIs determined by timing circuit 90 to one or moretachyarrhythmia detection interval zones, such as a tachycardiadetection interval zone and a fibrillation detection interval zone. RRIsfalling into a detection interval zone are counted by a respective VTinterval counter or VF interval counter and in some cases in a combinedVT/VF interval counter included in tachyarrhythmia detector 92.

When a VT, VF, or combined VT/VF interval counter reaches a thresholdcount value, often referred to as “number of intervals to detect” or“NID,” a ventricular tachyarrhythmia may be detected by control circuit80. Tachyarrhythmia detector 92 may be configured to perform othersignal analysis for determining if other detection criteria aresatisfied before detecting VT or VF when an NID is reached however. Forexample, cardiac signal analysis may be performed to determine if R-wavemorphology criteria, onset criteria, and noise and oversensing rejectioncriteria are satisfied in order to determine if the VT/VF detectionshould be made or withheld. As disclosed herein, tachyarrhythmiadetector 92 may withhold the VT or VF detection when an NID is reachedif analysis of cardiac signal waveform features indicates that therhythm is an SVT rhythm. In particular, if criteria indicating that afast ventricular rate is likely a rapidly conducted AF rhythm, a VT orVF detection based on the NID being reached may be rejected.

To support additional cardiac signal analyses performed bytachyarrhythmia detector 92, sensing circuit 86 may pass a digitizedcardiac electrical signal to control circuit 80. A cardiac electricalsignal from the selected sensing channel, e.g., from first sensingchannel 83 and/or the second sensing channel 85, may be passed through afilter and amplifier, provided to a multiplexer and thereafter convertedto multi-bit digital signals by an analog-to-digital converter, allincluded in sensing circuit 86, for storage in memory 82. This digitizedcardiac electrical signal or segments thereof may be used by controlcircuit 80 to analyze amplitude and/or morphology information for use inSVT discrimination as described below.

Memory 82 may include read-only memory (ROM) in which stored programscontrolling the operation of the control circuit 80 reside. Memory 82may further include random access memory (RAM) or other memory devicesconfigured as a number of recirculating buffers capable of holding aseries of measured RRIs, counts or other data for analysis by thetachyarrhythmia detector 92. Memory 82 may be configured to store apredetermined number of cardiac electrical signal segments incirculating buffers under the control of control circuit 80. Forinstance, up to eight cardiac electrical signal segments eachcorresponding to an R-wave sensed event signal may be stored in memory82. Additionally or alternatively, features derived from each of up toeight cardiac signal segments that each correspond to an R-wave sensedevent signal may be buffered in memory 82 for use in SVT discriminationas described below.

Therapy delivery circuit 84 includes charging circuitry, one or morecharge storage devices such as one or more high voltage capacitorsand/or low voltage capacitors, and switching circuitry that controlswhen the capacitor(s) are discharged across a selected pacing electrodevector or CV/DF shock vector. Charging of capacitors to a programmedpulse amplitude and discharging of the capacitors for a programmed pulsewidth may be performed by therapy delivery circuit 84 according tocontrol signals received from control circuit 80. Timing circuit 90 ofcontrol circuit 80 may include various timers or counters that controlwhen ATP or other cardiac pacing pulses are delivered. For example,timing circuit 90 may include programmable digital counters set by amicroprocessor of the control circuit 80 for controlling the basicpacing time intervals associated with various pacing modes or ATPsequences delivered by ICD 14. The microprocessor of control circuit 80may also set the amplitude, pulse width, polarity or othercharacteristics of the cardiac pacing pulses, which may be based onprogrammed values stored in memory 82.

In response to detecting VT or VF, control circuit 80 may controltherapy delivery circuit 84 to deliver therapies such as ATP and/orCV/DF therapy. Therapy can be delivered by initiating charging of highvoltage capacitors via a charging circuit, both included in therapydelivery circuit 84. Charging is controlled by control circuit 80, whichmonitors the voltage on the high voltage capacitors passed to controlcircuit 80 via a charging control line. When the voltage reaches apredetermined value set by control circuit 80, a logic signal isgenerated on a capacitor full line and passed to therapy deliverycircuit 84, terminating charging. A CV/DF pulse is delivered to theheart under the control of the timing circuit 90 by an output circuit oftherapy delivery circuit 84 via a control bus. The output circuit mayinclude an output capacitor through which the charged high voltagecapacitor is discharged via switching circuitry, e.g., an H-bridge,which determines the electrodes used for delivering the cardioversion ordefibrillation pulse and the pulse wave shape. In some examples, thehigh voltage therapy circuit configured to deliver CV/DF shock pulsescan be controlled by control circuit 80 to deliver pacing pulses, e.g.,for delivering ATP or post shock pacing pulses. In other examples,therapy delivery circuit 84 may include a low voltage therapy circuitfor generating and delivering relatively lower voltage pacing pulses fora variety of pacing needs. Therapy delivery and control circuitrygenerally disclosed in any of the above-incorporated patents may beimplemented in ICD 14.

It is recognized that aspects of the methods disclosed herein may beimplemented in an implantable medical device that is used for monitoringcardiac electrical signals by sensing circuit 86 and control circuit 80without having therapy delivery capabilities or in an implantablemedical device that monitors cardiac electrical signals and deliverscardiac pacing therapies by therapy delivery circuit 84, without highvoltage therapy capabilities, such as cardioversion/defibrillation shockcapabilities or vice versa.

Control parameters utilized by control circuit 80 for detecting cardiacarrhythmias and controlling therapy delivery may be programmed intomemory 82 via telemetry circuit 88. Telemetry circuit 88 includes atransceiver and antenna for communicating with external device 40 (shownin FIG. 1A) using RF communication or other communication protocols asdescribed above. Under the control of control circuit 80, telemetrycircuit 88 may receive downlink telemetry from and send uplink telemetryto external device 40. In some cases, telemetry circuit 88 may be usedto transmit and receive communication signals to/from another medicaldevice implanted in patient 12.

FIG. 4 is a diagram of circuitry included in first sensing channel 83and second sensing channel 85 of sensing circuit 86 according to oneexample. First sensing channel 83 may be selectively coupled viaswitching circuit 61 to a first sensing electrode vector includingelectrodes carried by extra-cardiovascular lead 16 as shown in FIGS.1A-1B or 2A-2C for receiving a first cardiac electrical signal. Firstsensing channel 83 may be coupled to a sensing electrode vector that isa short bipole, having a relatively shorter inter-electrode distance orspacing than the second electrode vector coupled to second sensingchannel 85. For example, the first sensing electrode vector may includepace/sense electrodes 28 and 30. In other examples, the first sensingelectrode vector coupled to sensing channel 83 may include adefibrillation electrode 24 and/or 26, e.g., a sensing electrode vectorbetween pace/sense electrode 28 and defibrillation electrode 24 orbetween pace/sense electrode 30 and either of defibrillation electrodes24 or 26. In still other examples, the first sensing electrode vectormay be between defibrillation electrodes 24 and 26.

In some patients, a bipole between electrodes carried by lead 16 mayresult in patient body posture dependent changes in the cardiacelectrical signal as the sensing vector of the bipole relative to thecardiac axis changes with changes in patient body posture or bodymotion. Accordingly, the sensing electrode vector coupled to the firstsensing channel 83 may include housing 15 and any of the electrodes 24,26, 28 and 30 carried by lead 16. A relatively longer bipole includinghousing 15 and a lead-based electrode may be less sensitive topositional changes. Cardiac electrical signals received viaextra-cardiovascular electrodes may be more influenced by positionalchanges of the patient than electrodes carried by transvenous leads. Theamplitude, polarity, and wave shape of R-waves may change, for example,as patient posture changes. As a result, R-wave morphology analysisperformed to discriminate between SVT, such as rapidly conducted AF, andVT/VF may lead to false VT/VF detection when R-wave amplitude and/ormorphology has changed due to positional changes of the patient. Thetechniques disclosed herein may be used to detect and discriminate SVTto avoid false detection of VT and VF and unnecessary electricalstimulation therapies even when patient posture changes cause changes inQRS amplitude and morphology.

Sensing circuit 86 includes a second sensing channel 85 that receives asecond cardiac electrical signal. The second cardiac electrical signalmay be received from a second sensing vector, for example from a vectorthat includes a pace/sense electrode 28 or 30 paired with housing 15.Second sensing channel 85 may be selectively coupled to other sensingelectrode vectors, which may form a relatively long bipole having aninter-electrode distance or spacing that is greater than the sensingelectrode vector coupled to first sensing channel 83 in some examples.As described below, the second cardiac electrical signal received bysecond sensing channel 85 via a long bipole may be used by controlcircuit 80 for morphology analysis and for determining cardiac signalsegment features for use in SVT discrimination. In other examples, anyvector selected from the available electrodes, e.g., electrodes 24, 26,28, 30 and/or housing 15, may be included in a sensing electrode vectorcoupled to second sensing channel 85. The sensing electrode vectorscoupled to first sensing channel 83 and second sensing channel 85 aretypically different sensing electrode vectors, which may have no commonelectrodes or only one common electrode but not both. In some instances,however, first sensing channel 83 and second sensing channel 85 may becoupled to the same sensing electrode vector. In this case, some of thecircuitry for first and second sensing channels may be common, such aspre-filter and pre-amplifier circuits 62, 72, ADCs 63, 73 and filters64, 74.

In the illustrative example shown in FIG. 4, the electrical signalsdeveloped across a first sensing electrode vector are received bysensing channel 83 and electrical signals developed across a secondsensing electrode vector are received by sensing channel 85. The cardiacelectrical signals are provided as differential input signals to thepre-filter and pre-amplifiers 62 and 72, respectively, of first sensingchannel 83 and second sensing channel 85. Non-physiological highfrequency and DC signals may be filtered by a low pass or bandpassfilter included in each of pre-filter and pre-amplifiers 62 and 72, andhigh voltage signals may be removed by protection diodes included inpre-filter and pre-amplifiers 62 and 72. Pre-filter and pre-amplifiers62 and 72 may amplify the pre-filtered signal by a gain of between 10and 100, in one example a gain of 17.5, and may convert the differentialsignal to a single-ended output signal that is passed toanalog-to-digital converter (ADC) 63 in first sensing channel 83 and toADC 73 in second sensing channel 85. Pre-filter and preamplifiers 62 and72 may provide anti-alias filtering and noise reduction prior todigitization.

ADC 63 and ADC 73, respectively, convert the first cardiac electricalsignal from an analog signal to a first digital bit stream and thesecond cardiac electrical signal to a second digital bit stream. In oneexample, ADC 63 and ADC 73 may be sigma-delta converters (SDC), butother types of ADCs may be used. In some examples, the outputs of ADC 63and ADC 73 may be provided to decimators (not shown), which function asdigital low-pass filters that increase the resolution and reduce thesampling rate of the respective first and second cardiac electricalsignals.

In first sensing channel 83, the digital output of ADC 63 is passed tofilter 64 which may be a digital bandpass filter having a bandpass ofapproximately 10 Hz to 30 Hz for passing cardiac electrical signals suchas R-waves typically occurring in this frequency range. The bandpassfiltered signal is passed from filter 64 to rectifier 65 then to R-wavedetector 66. In some examples, the filtered, digitized cardiacelectrical signal 69 from sensing channel 83, e.g., output of filter 64or rectifier 65, may be stored in memory 82 for signal processing bycontrol circuit 80 for use in detecting and discriminatingtachyarrhythmia episodes. The output signal 69 may be passed fromsensing circuit 86 to memory 82 under the control of control circuit 80for storing segments of the first cardiac electrical signal in temporarybuffers of memory 82. For example, timing circuit 90 of control circuit80 may set a time interval or number of sample points relative to anR-wave sensed event signal 68 received from R-wave detector 66, overwhich the segment of the first digitized cardiac electrical signal 69 isstored in memory 82. The buffered, first cardiac electrical signalsegment may be analyzed by control circuit 80 on a triggered, as neededbasis for determining cardiac signal segment features for discriminatingSVT and withholding an interval-based VT or VF detection, even whenother R-wave morphology analysis meets VT/VF detection criteria. Asdescribed below, analysis of the first cardiac electrical signal segmentmay be performed for determining if a pause in fast RRIs is detected asevidence of SVT, e.g., as evidence of a rapidly conducted AF rhythm. Ifsuch a pause is detected, tachyarrhythmia detector 92 of control circuit80 may withhold or delay detection of VT/VF.

R-wave detector 66 may include an auto-adjusting sense amplifier,comparator and/or other detection circuitry that compares the filteredand rectified first cardiac electrical signal to an R-wave sensingthreshold in real time and produces an R-wave sensed event signal 68when the cardiac electrical signal crosses the R-wave sensing thresholdoutside of a post-sense blanking period.

The R-wave sensing threshold, controlled by sensing circuit 86 and/orcontrol circuit 80, may be a multi-level sensing threshold as disclosedin pending U.S. Publication No. 2017/0312534 (Cao, et al.), incorporatedherein by reference in its entirety. Briefly, the multi-level sensingthreshold may have a starting sensing threshold value held for a timeinterval, which may be equal to a tachycardia detection interval orexpected R-wave to T-wave interval, then drops to a second sensingthreshold value held until a drop time interval expires, which may be 1to 2 seconds long. The sensing threshold drops to a minimum sensingthreshold, which may correspond to a programmed sensitivity, after thedrop time interval. In other examples, the R-wave sensing threshold usedby R-wave detector 66 may be set to a starting value based on themost-recently sensed R-wave peak amplitude and decay linearly orexponentially over time until reaching a minimum sensing threshold. Thetechniques described herein are not limited to a specific behavior ofthe sensing threshold. Instead, other decaying, step-wise adjusted orother automatically adjusted sensing thresholds may be utilized.

The second cardiac electrical signal, digitized by ADC 73 of sensingchannel 85, may be passed to filter 74 for bandpass filtering. In someexamples, filter 74 is a wideband filter for passing frequencies from 1to 30 Hz or higher. In some examples, sensing channel 85 includes notchfilter 76. Notch filter 76 may be implemented in firmware or hardware toattenuate 50 Hz or 60 Hz electrical noise in the second cardiacelectrical signal. Cardiac electrical signals acquired usingextra-cardiovascular electrodes may be more susceptible to 50 to 60 Hzelectrical noise than transvenous or intra-cardiac electrodes, musclenoise and other EMI, electrical noise or artifacts. As such, notchfilter 76 may be provided to significantly attenuate the magnitude ofsignals in the range of 50-60 Hz with minimum attenuation of signals inthe range of approximately 1-30 Hz, corresponding to typical cardiacelectrical signal frequencies.

The output signal 78 of notch filter 76 may be passed from sensingcircuit 86 to memory 82 under the control of control circuit 80 forstoring segments of the second cardiac electrical signal 78 in temporarybuffers of memory 82. For example, timing circuit 90 of control circuit80 may set a time interval or number of sample points relative to anR-wave sensed event signal 68 received from first sensing channel 83,over which the second cardiac electrical signal 78 is stored in memory82. The buffered, second cardiac electrical signal segment may beanalyzed by control circuit 80 on a triggered, as needed basis fordetermining cardiac signal segment features for discriminating SVT andwithholding an interval-based VT or VF detection, even when other R-wavemorphology analysis meets VT/VF detection criteria. As described below,analysis of the second cardiac electrical signal segment may beperformed for determining if a pause in fast RRIs is detected asevidence of SVT, e.g., as evidence of a rapidly conducted AF rhythm. Ifsuch a pause is detected, tachyarrhythmia detector 92 of control circuit80 may withhold or delay detection of VT/VF.

Notch filter 76 may be implemented as a digital filter for real-timefiltering performed by firmware as part of sensing channel 85 or bycontrol circuit 80 for filtering the buffered digital output of filter74. In some examples, the output of filter 74 of sensing channel 85 maybe stored in memory 82 in time segments defined relative to an R-wavesensed event signal 68 prior to filtering by notch filter 76. Whencontrol circuit 80 is triggered to buffer and analyze segments of thesecond cardiac electrical signal, for example as described inconjunction with FIG. 7, the notch filter 76 may be applied to thesecond cardiac electrical signal before morphology analysis anddetermination of cardiac signal segment features used for SVTdiscrimination.

The configuration of sensing channels 83 and 85 shown in FIG. 4 isillustrative in nature and should not be considered limiting of thetechniques described herein. The sensing channels 83 and 85 of sensingcircuit 86 may include more or fewer components than illustrated anddescribed in FIG. 4. First sensing channel 83 may be configured todetect R-waves from a first cardiac electrical signal in real time,e.g., in hardware implemented components, based on crossings of anR-wave sensing threshold by the first cardiac electrical signal, andsecond sensing channel 85 may be configured to provide a second cardiacelectrical signal for storage in memory 82 for processing and analysisby control circuit 80 for determining if the signal waveform morphologycorresponding to a sensed R-wave in the first sensing channel isindicative of VT or VF or if the signal waveform features support an SVTdetection and withholding of VT or VF detection. In other examples, bothsensing channels 83 and 85 may be capable of sensing R-waves in realtime and/or both channels 83 and 85 may provide a digitized cardiacsignal for buffering in memory 82 for morphological signal analysisduring VT/VF detection algorithms. In still other examples, sensingcircuit 86 may include only a single sensing channel.

FIG. 5 is a flow chart 100 of a method performed by ICD 14 for detectinga pause in a fast ventricular rate. As described herein, the detectedpause may be evidence of a rapidly conducted AF rhythm, a type of SVTthat may be inadvertently detected as a ventricular tachyarrhythmia. AFthat is conducted to the ventricles results in short, irregular RRIsthat may be falsely detected as a ventricular tachyarrhythmia.Discrimination of rapidly conducted AF from a true ventriculartachyarrhythmia may be based on detecting a pause in the conductedrhythm. A pause in the conducted rhythm is a relatively long RRI duringa fast ventricular rate, which may occur when an atrial fibrillationwave is not conducted to the ventricles due to a state of refractorinessof the ventricular myocardium and inherent conduction delays in theintrinsic cardiac conduction system. By detecting a pause in theconducted AF rhythm, an improper ventricular tachyarrhythmia detectionmay be withheld.

At block 102, control circuit 80 establishes an SVT morphology templaterepresentative of the expected R-wave morphology during an SVT. The SVTmorphology template is therefore also referred to herein as an “R-wavetemplate.” The SVT morphology template may be established according totechniques generally disclosed in the above-incorporated U.S. Pat. No.6,393,316 (Gillberg, et al.), and in U.S. Pat. Pub. No. 2018/0303368(Zhang, et al.), incorporated herein by reference in its entirety. TheSVT morphology template represents the expected R-wave morphology duringa supraventricular rhythm, which may be a sinus rhythm or an atrialtachyarrhythmia that is conducted to the ventricles. The SVT morphologytemplate may be acquired during a slow, non-paced ventricular rhythm torepresent a QRS waveform arising from the sinus node and is notnecessarily acquired during supraventricular tachycardia. In otherexamples, the SVT morphology template may be acquired during sinustachycardia, for example during patient exercise. The SVT morphologytemplate represents the QRS waveform morphology expected when theventricular depolarization is conducted to the ventricles from the atriavia the intrinsic ventricular conduction system (e.g., His bundle andPurkinje fibers).

Control circuit 80 may be configured to analyze cardiac electricalsignals for detecting a pause in a conducted AF rhythm after firstdetermining that a fast ventricular rate is suspected. At block 104,control circuit 80 determines that a plurality of sensed ventricularevents meet a fast ventricular rate criteria. In one example, controlcircuit 80 may determine that the plurality of sensed ventricular eventsmeet the fast ventricular rate criteria in response to detecting apredetermined number of RRIs determined using the plurality of sensedventricular events are characterized as ventricular tachycardiadetection intervals (TDIs), a predetermined number of ventricularfibrillation detection intervals (FDIs), or a predetermined number of acombination of TDIs and FDIs. TDIs and FDIs are detected as RRIs thatare less than a respective TDI threshold and FDI threshold. For example,the TDI threshold may be programmable between 280 and 650 ms, e.g., setnominally to 360 ms. The FDI threshold may be programmable between 240ms and 400 ms, e.g., set nominally to 320 ms.

The RRIs compared to the TDI threshold and the FDI threshold aredetermined based on R-wave sensed event signals produced by sensingcircuit 86 in response to the first sensing channel 83 detecting anR-wave sensing threshold crossing by the first cardiac electrical signaloutside of a blanking period. The R-wave sensed event signal may bepassed to control circuit 80. In response to the R-wave sensed eventsignal, timing circuit 90 of control circuit 80 determines an RRI endingwith the current R-wave sensed event signal and beginning with the mostrecent preceding R-wave sensed event signal. The timing circuit 90 ofcontrol circuit 80 may pass the RRI timing information to thetachyarrhythmia detection circuit 92 which adjusts tachyarrhythmiainterval counters, e.g., VT, VF or VT/VF interval counters, based on theRRI.

If the RRI is shorter than the TDI threshold but longer than the FDIthreshold, i.e., if the RRI is in a VT detection interval zone, a VTinterval counter is increased. The VT interval counter may be configuredto count consecutive VT intervals for detecting VT in which case the VTinterval counter may be reset to zero if the RRI is longer than the TDI.In other instances, the VT interval counter may be configured to countVT intervals that occur within a particular time window, e.g., within aparticular period of time or within a particular number of beats or RRIintervals (X of Y). If the RRI is shorter than the FDI, the VF counteris increased. The VF counter may be a probabilistic VF counter thatcounts VF intervals in an X of Y manner such that VF may be detectedwhen a threshold number of VF intervals are detected which are notrequired to be consecutive. In some examples, a combined VT/VF intervalcounter is increased if the RRI is less than the TDI. In some cases, VTdetection may be disabled such that the suspected fast rate is detectedat block 104 based only on a predetermined number of FDIs.

After updating the tachyarrhythmia interval counters, tachyarrhythmiadetector 92 compares the VT and VF interval counter values (andoptionally a combined VT and VF interval counter) to a suspected fastrate threshold at block 104. The suspected fast rate threshold valuesare less than the respective VT NID and VF NID required in order todetect VT or VF, respectively. Different fast heart rate thresholds maybe applied to the VT interval counter and the VF interval counter. Thesuspected fast rate threshold may be a count of at least two TDIs or acount of at least three FDIs in one example. The fast heart ratethreshold may be a value of one or more. In other examples, the fastheart rate threshold is a higher number, for example five or higher, butmay be less than the NID required to detect VT or VF.

If a VT interval counter, a VF interval counter, or a combined VT and VFinterval counter reaches the fast rate threshold set for the respectivecounter, “yes” branch of block 104, control circuit 80 detects asuspected fast ventricular rate and may enable analysis on an event byevent basis for detecting a pause in the fast rate of sensed R-wavesthat is evidence of a rapidly conducted AF rhythm. In response todetecting the suspected fast rate of ventricular events at block 104,the RRI ending with the currently sensed R-wave is compared to a pausethreshold interval at block 106. If the RRI is not greater than thepause threshold interval, a pause is not detected. Control circuit 80continues to compare RRIs to the pause threshold interval on a beat bybeat basis as long as the suspected fast rate criteria are met at block104. The RRI determined at block 106 and compared to the pause thresholdinterval may be based on R-wave sensed event signals received from thefirst sensing channel 83 based on R-wave sensing threshold crossings ofthe first cardiac electrical signal. In other examples, however, the RRIdetermined at block 106 and compared to the pause threshold interval maybe based on a cardiac electrical signal received by the second sensingchannel 85.

The pause threshold interval may be set to be equal to the TDI when VTdetection is enabled. If VT detection is not enabled, the pausethreshold interval may be set to be equal to the FDI. In some examples,control circuit 80 may establish an SVT limit as a minimum median RRIfor which an SVT can be detected or a VT/VF detection can be withheld.The SVT limit may be established as a programmed parameter stored inmemory 82. In some examples, if the median RRI out of a sequence of apredetermined number of RRIs is less than the SVT limit, SVT detectioncannot be made and cannot cause withholding of a VT or VF detection. Forexample, the SVT limit may be programmable and set between 210 and 310ms. A nominal SVT limit may be 260 ms. If the median RRI determined fromthe most recent 12 RRIs, or other predetermined number of RRIs, is lessthan the SVT limit, an SVT is not detected, and any SVT detectionrelated criteria are not used to withhold a VT or VF detection. In theprocess shown in FIG. 5, the SVT limit may be used in setting the pausethreshold interval. The pause threshold interval may be set to thelargest of the SVT limit and the TDI when VT detection is enabled or thelargest of the SVT limit and the FDI plus an offset interval, e.g., FDIplus 20 ms, when VT detection is disabled.

If the current RRI is greater than the pause threshold interval at block106, the control circuit 80 advances to block 108 to determine amorphology match score for each of the starting and trailing R-waves ofthe current RRI that is greater than the pause threshold interval. Inone example, the QRS signal is buffered from a second cardiac electricalsignal received from the second sensing channel 85 and notch filteredprior to determining the morphology match score. An R-wave sensed eventsignal received from the first sensing channel 83 may be used as atiming marker for selecting the beginning and ending sample pointsstored from the second cardiac electrical signal for buffering a cardiacsignal segment corresponding to each of the starting and trailingR-waves of the RRI greater than the pause threshold interval. In thisway, the first sensing channel 83 may be used for sensing R-waves, andthe second sensing channel 85 may be used for acquiring cardiac signalsegments from a different sensing vector that are compared to the SVTmorphology template for determining a morphology match score at block108. In another example, both the sensing R-waves and acquiring cardiacsignal segments that are compared to the SVT morphology template fordetermining a morphology match score may be from the same sensingvector, such as the first sensing vector. In this case, the secondcardiac electrical signal may be a segment of the cardiac electricalsignal occurring after the detection of the fast ventricular rate.

The morphology match score may be determined using a wavelet transformanalysis, e.g., as described in the above-incorporated Gillberg patent.In one example, 48 sample points of the digitized, notch-filteredcardiac electrical signal buffered from the second cardiac electricalsignal may be processed to determine Haar wavelet-domain coefficients.The coefficients are compared to corresponding coefficients of thepreviously established SVT morphology template. Differences between thecoefficients are determined and summed to determine the morphologymatching score as percentage with a maximum possible score of 100.

Control circuit 80 compares morphology match scores of the starting andtrailing R-waves of the RRI greater than the pause threshold interval toan adjusted match threshold at block 110. The morphology match thresholdapplied at block 110 is referred to as an “adjusted” match thresholdbecause it may be different than a match threshold used bytachyarrhythmia detector 92 for identifying SVT beats as a part of otherparts of the overall VT/VF detection algorithm. For example, when asuspected fast rate is detected, a match score may be determined bycomparing R-wave morphology on a beat-by-beat basis to detect SVT beats.If a predetermined number of SVT beats are detected out of the mostrecent sensed R-waves, for example if 3 or more SVT beats are detectedout of the most recent 8 sensed R-waves based on comparing themorphology matching score to the match threshold, an SVT rejection rulemay be enabled so that if the VT or VF NID is reached based on RRIs, theVT or VF detection is withheld. A process for detecting SVT beats on abeat-by-beat basis for determining the status of an SVT rejection ruleafter suspected fast rate criteria are met is generally described in theabove-incorporated U.S. Pat. Pub. No. 2018/0028087 and in U.S. Pat. Pub.No. 2018/0028085 (Zhang, et al.). The match threshold used to detect andcount SVT beats may be different than the match threshold applied atblock 110 for detecting a pause in a conducted AF rhythm. Detection ofSVT beats using a first match threshold may be used to set a beatmorphology rejection rule. The beat morphology rejection rule may besatisfied when a minimum number of morphology match scores out of apredetermined number of most recent morphology match scores exceed thefirst match score threshold in one example. For example, if at leastthree out of eight of the most recent morphology match scores exceed amatch score threshold of 49, 61, 70 or other score threshold (out of apossible score of 100), the beat morphology rejection rule is satisfied.A relatively high match score, exceeding a selected match scorethreshold, indicates that an unknown beat during a fast rhythm matchesthe known SVT morphology template and is therefore an R-wave conductedfrom the atria rather than a VT or VF beat originating in theventricles.

In order to detect a pause that is evidence of a rapidly conducted AFrhythm, control circuit 80 may determine an adjusted match threshold atblock 110 and compare the adjusted match threshold to the morphologymatch score of the starting and trailing R-waves of each RRI greaterthan the pause threshold interval at block 110. The adjusted matchthreshold is adjusted from the first match threshold used to detect SVTbeats based on QRS morphology. The adjusted match threshold may bedecreased from the first, SVT beat match threshold by a predeterminedpercentage or decrement to allow an SVT morphology template match at aslightly lower match score than that required to detect an SVT beat. Insome examples, the percentage used to adjust the first match thresholdfor detecting an SVT beat to an adjusted match threshold used for pausedetection may be dependent on the programmed first match threshold. Forexample, the percentage may be scaled so that a higher percentage isused to adjust the first match threshold set to a higher value and alower percentage is used to adjust the first match threshold set to arelatively lower value.

To illustrate, if the first, SVT beat detection match threshold is setto 70 or higher, it may be adjusted by decreasing it by 10% to obtainthe adjusted, pause detection match threshold. If the first SVT beatdetection match threshold is set to 61 or higher but less than 70,control circuit 80 may be decrease the first match threshold by 7% toobtain the adjusted match threshold for pause detection at block 110. Ifthe first SVT beat detection match threshold is set to less than 61, itis decreased by only 4% to obtain the adjusted pause detection matchthreshold. In other examples, the adjusted pause detection matchthreshold may be a fixed decrement below the first match threshold usedfor SVT beat detection, which may or may not be scaled relative to thevalue of the first match threshold.

If one or both of the starting and trailing R-waves defining the RRIhave a morphology match score that is less than the adjusted matchthreshold at block 110, “no” branch of block 110, a pause is notdetected. The process returns to block 104. If both of the starting andtrailing R-waves defining the RRI have a morphology match score that isgreater than or equal to the adjusted match threshold at block 110,control circuit 80 may detect a pause as evidence of a conducted AFrhythm. In the example of FIG. 5, however, additional criteria fordetecting the pause are required to be met. These additional criteriamay be used in some examples. In other examples, the method may stopafter detecting the pause based on the RRI being greater than the pauseinterval threshold and both of the starting and trailing R-wavesdefining the RRI having a morphology match score that is greater than orequal to the adjusted match threshold.

At block 112, control circuit 80 determines the R-wave amplitudes of thestarting and trailing R-waves. The R-wave amplitudes may be determinedfrom the first cardiac electrical signal. In other instances, the R-waveamplitudes may be determined from the second cardiac electrical signalsensed by second sensing channel 85. If the absolute difference betweenthe R-wave amplitudes (R2−R1 where R2 is the trailing R-wave and R1 isthe starting R-wave of the RRI that is greater than the pause thresholdinterval) is less than a difference threshold at block 114, controlcircuit 80 detects a pause at block 116. The difference threshold may bepredefined or set based on the amplitude of either the starting or thetrailing R-wave. In one example, the difference threshold is set to onehalf of the trailing R-wave amplitude.

The pause detected at block 116 by control circuit 80 based on the firstand, in some instances, second cardiac electrical signal analysis may beevidence of a rapidly conducted AF rhythm. As described below, controlcircuit 80 may track the number of pauses detected over a predeterminednumber of sensed R-waves as long as the suspected fast rate criteria(block 104) are satisfied. If the NID for VT or VF detection is reached,at least one detected pause may cause control circuit 80 to withhold theVT or VF detection. In other instances, control circuit may need todetect more than one pause to withhold VT or VT detection.

FIG. 6 is a conceptual diagram 200 of a pause 201 in a fast rate of thesensed cardiac events that may be detected by control circuit 80. Afirst cardiac electrical signal 202 represents a filtered and rectifiedsignal produced by the first sensing channel 83, and passed to R-wavedetector 66, for producing R-wave sensed event signals 207. The secondcardiac electrical signal 212 represents a signal produced by the secondsensing channel 85, such as cardiac electrical signal 78 of FIG. 4. Inother instances, second cardiac electrical signal 212 may represent asignal produced by the first sensing channel 83, such as cardiacelectrical signal 69 output from rectifier 65 or a cardiac electricalsignal output from filter 64.

Control circuit 80 determines RRIs 203 and 204 between consecutiveR-wave sensed event signals 207. If the suspected fast heart ratecriteria become satisfied, e.g., based on a threshold count of VT and/orVF intervals, as described above, control circuit 80 compares each RRIto a pause threshold interval 205 as long as the suspected fast heartrate criteria are satisfied. The first RRI 203 shown in FIG. 6 isshorter than the pause threshold interval 205. In some examples, nofurther analysis of the first and second cardiac signals 202 and 212 isperformed for detecting a pause if the RRI 203 is less than the pausethreshold interval 205.

The next RRI interval 204 is longer than the pause threshold interval205. In response to the RRI interval 204 being longer than the pausethreshold interval 205, control circuit 80 may determine the R-waveamplitude 206 of the starting R-wave (R1) of RRI 204 and the R-waveamplitude 208 of the trailing R-wave (R2) of RRI 204. The absolutedifference between R1 amplitude 206 and R2 amplitude 208 is compared toa difference threshold. The difference threshold may be a defined valueor set to a percentage of the R1 or R2 amplitude. In one example, thedifference threshold is set to half of the R2 amplitude 208. If the R1and R2 amplitudes 206 and 208 are within a difference threshold of eachother, criteria for detecting RRI 204 as a pause in a suspected fastheart rate may be satisfied. R-wave amplitudes 206 and 208 aredetermined based on first cardiac electrical signal 202 in this example.However, in other examples, R-wave amplitudes 206 and 208 may bedetermined based on the digitized and filtered signals of either firstsensing channel 83 (e.g., output signal 69 or an output signal of filter64) or second sensing channel 85 (e.g., output signal 78).

In some examples, control circuit 80 may additionally or alternativelydetermine a morphology match score of the starting R-wave 218 and thetrailing R-wave 220 of the second cardiac electrical signal 212 inresponse to RRI 204 being greater than the pause threshold interval 205.After the suspected fast rate criteria are met, e.g., when the TDI countreaches two or the FDI count reaches three, cardiac signal segments fromthe second cardiac electrical signal 212 are buffered over a timeinterval 214 set in response to each respective R-wave sensed eventsignal 207 from the first sensing channel 83. The cardiac signal segmentacquired over the time interval 214 and corresponding to the startingR-wave 218 may be processed to determine wavelet coefficients that arecompared to analogous wavelet coefficients of the SVT morphologytemplate 210 to determine a morphology match score. As described above,the SVT morphology template 210 is previously established to representthe QRS morphology during an SVT rhythm. The cardiac signal segment overthe time interval 214 and corresponding to the trailing R-wave 220 isalso processed to determine a morphology match score from a comparisonbetween the morphology of R-wave 220 and the SVT morphology template210. In other examples, control circuit 80 determines a morphology matchscore of the starting R-wave 218 and the trailing R-wave 220 based onthe digitized and filtered signals of the first sensing channel 83(e.g., an output signal of filter 64).

The morphology match scores of R-wave 218 and R-wave 220 may each becompared to an adjusted match threshold as described above. If bothmatch scores are greater than or equal to the adjusted match threshold,and all other criteria for detecting a pause are satisfied, RRI 204 isdetected by control circuit 80 as a pause 201 in the fast heart rate. Insome examples, control circuit 80 includes a counter configured to countthe number of pause detections out of the most recent Y RRIs, forexample out of the most recent twenty RRIs. If the VT NID or VF NID isreached and at least one pause has been detected in the most recenttwenty RRIs, the VT or VF detection may be withheld. As described below,other criteria may be required to be satisfied in addition to athreshold number of detected pauses in order to withhold a VT or VFdetection based on evidence of a rapidly conducted AF rhythm.

Control circuit 80 may detect the pause using the RRI interval lengthalone in some instances. In other instances, control circuit 80 maydetect the pause using the RRI interval length in conjunction with oneor both of the amplitude or morphology associated with the starting andtrailing R-waves meeting the pause criteria.

FIG. 7 is a flow chart 300 of a method for detecting ventriculartachyarrhythmias according to one example using the rapidly conducted AFdetection techniques disclosed herein. At block 302, a sensing electrodevector is selected by sensing circuit 86 for receiving a cardiacelectrical signal by first sensing channel 83 used for sensing R-waves.The first sensing vector selected at block 302 for obtaining the cardiacelectrical signal used for sensing R-waves may be a relatively shortbipole, e.g., between electrodes 28 and 30 or between electrodes 28 and24 of lead 16 or other electrode combinations as described above. Thefirst sensing vector may be a vertical sensing vector (with respect toan upright or standing position of the patient) or approximately alignedwith the cardiac axis for maximizing the amplitude of R-waves in thecardiac electrical signal for reliable R-wave sensing. In otherexamples, the first sensing vector may be a vector between one electrodecarried along the distal portion 25 of lead 16 and the ICD housing 15(shown in FIG. 1A). The sensing circuit 86 selects a second sensingvector at block 304 for receiving the cardiac signal that is bufferedfor obtaining cardiac signal segments for morphology analysis and SVTdiscrimination. As described above, in some instances, the secondsensing vector is the same as the first sensing vector.

Sensing circuit 86 may produce an R-wave sensed event signal at block306 in response to the first sensing channel 83 detecting the cardiacelectrical signal crossing an R-wave sensing threshold outside of ablanking period. The R-wave sensed event signal may be passed to controlcircuit 80. In response to the R-wave sensed event signal, timingcircuit 90 of control circuit 80 determines an RRI at block 310 endingwith the current R-wave sensed event signal and beginning with the mostrecent preceding R-wave sensed event signal. The timing circuit 90 ofcontrol circuit 80 may pass the RRI timing information to thetachyarrhythmia detection circuit 92 which adjusts tachyarrhythmiainterval counters, e.g., VT interval counter, VF interval counter and/orcombined VT/VF interval counter, at block 312 as needed based on theRRI.

For example, if the RRI is shorter than the TDI and longer than the FDI,a VT interval counter is increased at block 312. The VT interval countermay be reset to zero if the RRI is longer than the TDI. If the RRI isshorter than the FDI, the VF counter is increased. A combined VT/VFinterval counter may be increased if the RRI is less than the TDI. Afterupdating the tachyarrhythmia interval counters at block 312,tachyarrhythmia detector 92 compares the VT and VF interval countervalues to a suspected fast rate threshold at block 314. The suspectedfast rate threshold is less than number of TDIs and FDIs required todetect VT and VF, i.e., less than the respective VT NID and VF NID. If athreshold number of short RRIs are counted, a fast ventricular rate issuspected but several more TDIs or FDIs are required before a VT or VFdetection can be made. As described above, the fast rate threshold maybe a count of two on the VT interval counter and a count of three on theVF interval counter in one example.

If a VT or VF detection interval counter has reached a fast ratethreshold, “yes” branch of block 314, control circuit 80 enables cardiacsignal segment buffering at block 404. In this example, thedetermination of parameters for detecting a pause in the suspected fastventricular rate evidencing a rapidly conducted AF rhythm may beperformed on an event-by-event basis only after at least one of the VTor VF interval counter values (or a combined VT/VF interval counter) hasreached the suspected fast rate threshold.

If the suspected fast rate threshold is not reached by any of thetachyarrhythmia interval counters at block 314, the control circuit 80returns to block 306 and waits for the next R-wave sensed event signal.Analysis of cardiac signal segments from the second cardiac electricalsignal need not be performed for discriminating between SVT and VT/VFuntil at least a threshold number of VT or VF intervals is counted inanticipation of an NID being reached. In this way, control circuit 80makes a determination of whether a pause is detected as evidence of aconducted AF rhythm only when a fast ventricular rate is suspected toconserve processing power requirements. Control circuit 80 may startanalyzing pause detection parameters when the suspected fast ratethreshold is reached, before the NID is reached, so that the detectionof a pause indicative of a conducted AF rhythm can be made by the timethe NID is reached. The VT or VF detection may be withheld based onpause detection, or the VT or VF detection is made without delay in theabsence of pause detection.

As long as the suspected fast rate threshold is satisfied at block 314,control circuit 80 enables buffering of cardiac signal segments from thesecond cardiac electrical signal at block 404. In response to eachR-wave sensed event signal produced by the first sensing channel 83 atblock 306, control circuit 80 buffers the cardiac electrical signalreceived by the second sensing channel 85 over a predetermined timeinterval or number of sample points for a given sampling rate.

As illustrated in FIG. 6, a digitized segment of the cardiac electricalsignal received by the second sensing channel 85 may be buffered over atime segment defined relative to the sample point time of the R-wavesensed event signal received from the first sensing channel 83. Thedigitized segment may be 100 to 500 ms long, for instance. In oneexample, the buffered segment of the second cardiac electrical signal isat least 48 sample points obtained at a sampling rate of 256 Hz, orapproximately 188 ms, of which 24 sample points may precede and includethe sample point at which the R-wave sensed event signal was receivedand 24 sample points may extend after the sample point at which theR-wave sensed event signal was received. In other examples, the cardiacelectrical signal segment may be buffered at block 404 over a longertime interval for use in other cardiac signal analyses performed todetect noise in the cardiac signal, T-wave oversensing, or other sensingissues that may lead to a false VT or VF detection.

The buffered cardiac signal segment may be notch filtered at block 406.The notch filter applied at block 406 may correspond to the filterdescribed in provisional U.S. Patent Pub. No. 2018/0028087, incorporatedherein by reference in its entirety. The notch filtering performed atblock 406 significantly attenuates 50-60 Hz electrical noise, musclenoise, other EMI, and other noise/artifacts in the stored cardiac signalsegment from the second cardiac electrical signal.

In one example, notch filtering performed at block 406 is implemented infirmware as a digital integer filter. The output of the digital notchfilter may be determined by firmware implemented in the second sensingchannel 85 according to the equation Y(n)=(x(n)+2*x(n−2)+x(n−4))/4,where x(n) is the amplitude of the nth sample point of the digitalsignal received by the notch filter 76 (FIG. 4), x(n−2) is the amplitudeof the n−2 sample point, and x(n−4) is the amplitude of the n−4 samplepoint for a sampling rate of 256 Hz. Y(n) is the amplitude of the nthsample point of the notch-filtered, digital second cardiac electricalsignal. At a frequency of 60 Hz, the attenuation of the magnitude ofY(n) is about −40 decibels (dB). At a frequency of 50 Hz, theattenuation is about '20 dB, and at 23 Hz, which may be typical of anR-wave of the cardiac electrical signal, the attenuation is limited toabout −3 dB. Notch filtering at block 406 may therefore provide highlyattenuated 50 and 60 Hz noise, muscle noise, other EMI, and otherelectrical noise/artifacts while passing lower frequency cardiacsignals, e.g., R-waves, in the cardiac electrical signal output ofsecond sensing channel 85.

The sample point numbers indicated in the equation above for determininga notch-filtered signal may be modified as needed when a differentsampling rate other than 256 Hz is used, and the resulting frequencyresponse may differ somewhat from the example given above. In otherexamples, other digital filters may be used for attenuation of 50 and 60Hz. For example, for a sampling rate of 256 Hz, a filtered signal Y(n)may be determined as Y(n)=(x(n)+x(n−1)+x(n−2)+x(n−3))/4 which may haverelatively less attenuation at 50 and 60 Hz but acts as a low-pass,notch filter with relatively greater attenuation at higher frequencies(greater than 60 Hz).

Under the control of control circuit 80, a predetermined number ofcardiac signal segments may be stored in memory 82 in a rolling,first-in-first-out buffer. For example, eight cardiac signal segmentsmay be buffered in memory 82 in a first-in-first-out manner as long asthe suspected fast rate is being detected based on a threshold VT or VFinterval counter value. At block 408, control circuit 80 may determineif a pause is detected using the techniques described above inconjunction with FIGS. 5 and 6. For example, in response to the RRIending with the currently sensed R-wave being greater than the pausethreshold interval, the absolute difference between the peak amplitudesof the starting and trailing R-waves of the current RRI are compared toa difference threshold, which is half of the trailing R-wave peakamplitude in one example. The R-wave amplitudes may be determined fromthe first cardiac signal received by the first sensing channel 83 or thesecond cardiac signal received by the second sensing channel 85. Inresponse to the R-wave amplitude difference being less than thedifference threshold, control circuit 80 may determine a morphologymatch score for each of starting and trailing R-waves from the buffered,notch-filtered second cardiac electrical signal segments at block 408.The morphology match scores are compared to a match threshold, which maybe adjusted from a higher match threshold value required for detectingSVT beats on a beat by beat basis as described above. A pause isdetected by control circuit 80 at block 407 in response to the currentRRI being greater than the pause threshold interval, both of thestarting and trailing R-waves of the current RRI matching the SVTmorphology template with a score greater than the adjusted matchthreshold, and the absolute difference between the peak amplitudes ofthe starting and trailing R-waves of the current RRI being less than thedifference threshold.

In response to detecting a pause, control circuit 80 may adjust a pausecounter or set a pause detection flag at block 410. In some examples,only one pause detection within the most recent twenty sensed R-waves isevidence of a rapidly conducted AF rhythm. In other examples, criteriafor detecting a conducted AF rhythm may require one or more pausesdetected within a predetermined number of the most recently sensedR-waves, which may be greater than or less than twenty sensed R-waves.

At block 412, control circuit 80 sets the status of an AF rejection rulebased on the pause counter or flag value. The AF rejection rule statusmay be satisfied based on a count of detected pauses, e.g., based on Npauses detected out of the most recent M sensed R-waves. In someexamples additional requirements must be satisfied in order for the AFrejection rule to be satisfied. For example, in order to reject a VT orVF detection based on pause detection(s) as evidence of a conducted AFrhythm, the median RRI (determined over the most recent 8 or otherselected number of RRIs) may be required to be greater than the SVTlimit in order to set the AF rejection rule as being satisfied.Additionally or alternatively, the VF interval counter may be requiredto be greater than a rapidly conducted AF threshold count. The VFinterval counter may be required to have reached a count of at least 6,for example, in order to set the AF rejection rule as being satisfied atblock 412.

If the criteria for detecting a conducted AF rhythm are satisfied,control circuit 80 sets the AF rejection rule at block 412 to indicatethat evidence of a conducted AF rhythm is detected. As an example if atleast one pause has been detected out of the most recent twenty sensedR-waves, the VF interval count value is greater than or equal to six,and the median RRI (e.g., determined from the most recent eight RRIs) isgreater than the programmed SVT limit, control circuit 80 may set the AFrejection rule to “true” by setting a flag or digital value stored inmemory 82 to a high value. If the conducted AF rhythm criteria are notsatisfied, e.g., less than N pauses detected out of the most recent Msensed R-waves, the median RRI is less than the SVT limit, and/or the VFinterval count has not yet reached a rapidly conducted AF thresholdcount, the AF rejection rule may be set to “false,” e.g., by setting theflag or digital value stored in memory 82 to zero or a digital lowvalue. The status of the AF rejection rule may be adjusted at block 412on a beat by beat basis as needed based on updates to the pausedetection counter (block 410) and other rapidly conducted AF rhythmcriteria as long as the suspected fast rate criteria are satisfied (atblock 314).

If an NID is not yet reached by one of the VT, VF or combined VT/VFinterval counters at block 316, control circuit 80 returns to block 306to sense the next R-wave, determine the next RRI for updating theinterval counters, and buffer the next segment of the second cardiacsignal if the suspected fast rate criteria are still satisfied at block314. The AF rejection rule status may be updated at block 412 based onthe analysis of the next RRI and associated R-wave amplitudes andmorphology match scores.

If an NID is reached at block 316 by one of the VT, VF or combined VT/VFinterval counters, control circuit 80 checks if a rejection rule issatisfied or set to “true” at block 318. If the AF rejection rule is setto “true,” VT or VF detection is withheld at block 324 even though theNID has been reached. No VT or VF therapy is delivered by therapydelivery circuit 84. Control circuit 80 advances to the next sensedR-wave to continue updating the VT and VF interval counters andanalyzing the next RRI and the associated R-wave amplitudes andmorphology match scores if the RRI is greater than the pause thresholdinterval to update the status of the AF rejection rule at block 412.

If the VT or VF NID is reached at block 316 and the AF rejection rule isset to “false” (“no” branch of block 318), the VT or VF episode may bedetected at block 320. The AF rejection rule may be one of multipleVT/VF rejection rules that may be set by control circuit 80. Otherexamples of rejection rules that may be set based at least in part on ananalysis of the second cardiac electrical signal after the suspectedfast rate criteria are satisfied are described in the above incorporatedpatent applications and may include a T-wave oversensing rejection rule,an SVT beat morphology rejection rule, a gross morphology rejectionrule, a noise rejection rule, as examples. Control circuit 80 detects VTor VF at block 320 in response to the respective NID being reachedwithout a rejection rule being satisfied. For instance, if at least theAF rejection rule is set to “false,” a VT or VF episode may be detectedat block 320 in response to the NID being reached. Control circuit 80controls therapy delivery circuit 84 to deliver a therapy at block 322,which may include one or more of ATP, a CV/DF shock, and/or post-shockpacing pulses in some examples.

Although the second cardiac electrical signal described in the exampleof FIG. 7 is that of the second sensing channel 85, in other instances

Thus, techniques for withholding a VT or VF detection based on detectingat least one pause indicating a high likelihood of a rapidly conductedAF rhythm are presented herein. It should be understood that, dependingon the example, certain acts or events of any of the methods describedherein can be performed in a different sequence, may be added, merged,or left out altogether (e.g., not all described acts or events arenecessary for the practice of the method). Moreover, in certainexamples, acts or events may be performed concurrently, e.g., throughmulti-threaded processing, interrupt processing, or multiple processors,rather than sequentially. In addition, while certain aspects of thisdisclosure are described as being performed by a single circuit or unitfor purposes of clarity, it should be understood that the techniques ofthis disclosure may be performed by a combination of units or circuitsassociated with, for example, a medical device.

In one or more examples, the functions described may be implemented inhardware, software, firmware, or any combination thereof. If implementedin software, the functions may be stored as one or more instructions orcode on a non-transitory computer-readable medium and executed by ahardware-based processing unit. Computer-readable media may includecomputer-readable storage media, which corresponds to a tangible,non-transitory medium such as data storage media (e.g., RAM, ROM,EEPROM, flash memory, or any other medium that can be used to storedesired program code in the form of instructions or data structures andthat can be accessed by a computer).

Instructions may be executed by one or more processors, such as one ormore digital signal processors (DSPs), general purpose microprocessors,application specific integrated circuits (ASICs), field programmablelogic arrays (FPGAs), or other equivalent integrated or discrete logiccircuitry. Accordingly, the term “processor,” as used herein may referto any of the foregoing structure or any other structure suitable forimplementation of the techniques described herein. Also, the techniquescould be fully implemented in one or more circuits or logic elements.

Thus, techniques have been presented in the foregoing description withreference to specific examples. It is to be understood that variousaspects disclosed herein may be combined in different combinations thanthe specific combinations presented in the accompanying drawings. It isappreciated that various modifications to the referenced examples may bemade without departing from the scope of the disclosure and thefollowing claims.

What is claimed is:
 1. A medical device comprising: a sensing circuitconfigured to receive a plurality of cardiac electrical signals andsense ventricular events from a first cardiac electrical signal of theplurality of cardiac electrical signals; and a control circuit coupledto the sensing circuit and configured to: determine a plurality ofventricular event intervals based on the ventricular events sensed bythe sensing circuit from the first cardiac electrical signal; determinethat a ventricular event interval of the plurality of ventricular eventintervals is greater than a pause threshold interval; determine that arule for withholding detecting a ventricular tachyarrhythmia is met atleast in response to determining the ventricular event interval that isgreater than the pause threshold interval; determine from the pluralityof ventricular event intervals that a threshold number oftachyarrhythmia intervals to detect a ventricular tachyarrhythmia isreached; and in response to determining that the rule is met, withholddetecting the ventricular tachyarrhythmia when the threshold number oftachyarrhythmia intervals to detect the ventricular tachyarrhythmia isreached.
 2. The medical device of claim 1, wherein the control circuitis configured to determine that the ventricular event interval that isgreater than the pause threshold interval occurs before the thresholdnumber of tachyarrhythmia intervals is reached.
 3. The medical device ofclaim 1, wherein the control circuit is further configured to: determinethat the threshold number of tachyarrhythmia intervals is reached bycomparing each of the plurality of ventricular event intervals to atachyarrhythmia detection interval; and set the pause threshold intervalto be at least equal to or greater than the tachyarrhythmia detectioninterval.
 4. The medical device of claim 1, wherein the control circuitis further configured to determine that the rule is met by: determiningfrom one of the plurality of cardiac electrical signals: a first featureof a starting ventricular event of the ventricular event interval thatis greater than the pause threshold interval; a second feature of atrailing ventricular event of the ventricular event interval that isgreater than the pause threshold interval; determining that the firstfeature and the second feature meet pause detection criteria; anddetermining that the rule is met in response to determining that thepause detection criteria are met.
 5. The medical device of claim 4,wherein the control circuit is further configured to: determine thefirst feature by determining a first peak amplitude ; determine thesecond feature by determining a second peak amplitude; determine thatthe difference between the first peak amplitude and the second peakamplitude is less than a difference threshold; and determine that thefirst feature and the second feature meet the pause detection criteriain response to the difference being less than the difference threshold.6. The medical device of claim 4, wherein the control circuit is furtherconfigured to: determine the first feature by determining a firstmorphology matching score between the starting ventricular event and amorphology template; determine the second feature by determining asecond morphology matching score between the trailing ventricular eventand the morphology template; determine that the first morphology matchscore and the second morphology match score are each greater than orequal to a match score threshold; and determine that the pause detectioncriteria are met in response to determining that each of the firstmorphology match score and the second morphology match score are greaterthan or equal to the match score threshold.
 7. The medical device ofclaim 1, wherein the control circuit is further configured to: determinethat at least a predetermined number of the threshold number oftachyarrhythmia intervals are less than or equal to a ventricularfibrillation detection interval; and determine that the rule is met inresponse to determining the ventricular event interval that is greaterthan the pause threshold interval and at least the predetermined numberof the threshold number of tachyarrhythmia intervals being less than orequal to the ventricular fibrillation detection interval.
 8. The medicaldevice of claim 1, wherein the control circuit is further configured to:determine a median interval of a predetermined number of the pluralityof ventricular event intervals; determine that the median interval isgreater than supraventricular tachyarrhythmia limit; and determine thatthe rule is met in response to determining the ventricular eventinterval that is greater than the pause threshold interval and themedian interval being greater than supraventricular tachyarrhythmialimit.
 9. The medical device of claim 1, wherein the control circuit isconfigured to determine that the rule for withholding detecting aventricular tachyarrhythmia is met by: determining that at least a firstpredetermined number of the threshold number of tachyarrhythmiaintervals are less than or equal to a ventricular fibrillation detectioninterval; determining that a median interval of a second predeterminednumber of the plurality of ventricular event intervals is greater than asupraventricular tachyarrhythmia limit; determining from the pluralityof cardiac electrical signals a difference between a first peakamplitude of a starting ventricular event of the ventricular eventinterval that is greater than the pause threshold interval a second peakamplitude of a trailing ventricular event of the ventricular eventinterval that is greater than the pause threshold interval; determiningthat the difference between the first peak amplitude and the second peakamplitude is less than a difference threshold; determining a firstmorphology matching score between the starting ventricular event and amorphology template; determining a second morphology matching scorebetween the trailing ventricular event and the morphology template; anddetermining that the first morphology match score and the secondmorphology match score are each greater than or equal to a match scorethreshold.
 10. The device of claim 1, further comprising a therapydelivery circuit; wherein the control circuit is configured to:determine from a next plurality of ventricular event intervals that thethreshold number of tachyarrhythmia intervals to detect the ventriculartachyarrhythmia is reached; determine that the rule for withholdingdetecting a ventricular tachyarrhythmia is not met based on the nextplurality of ventricular event intervals; and detect ventriculartachyarrhythmia in response to the threshold number of tachyarrhythmiaintervals determined from the next plurality of ventricular eventintervals and the rule not being met; and the therapy delivery circuitbeing configured to deliver a therapy in response to the ventriculartachyarrhythmia being detected.
 11. A method comprising: receiving aplurality of cardiac electrical signals; sensing ventricular events froma first cardiac electrical signal of the plurality of cardiac electricalsignals; determining a plurality of ventricular event intervals based onthe sensed ventricular events; determining that a ventricular eventinterval of the plurality of ventricular event intervals is greater thana pause threshold interval; determining that a rule for withholdingdetecting a ventricular tachyarrhythmia is met at least in response todetermining the ventricular event interval that is greater than thepause threshold interval; determining from the plurality of ventricularevent intervals that a threshold number of tachyarrhythmia intervals todetect a ventricular tachyarrhythmia is reached; and in response todetermining that the rule is met, withholding detecting the ventriculartachyarrhythmia when the threshold number of tachyarrhythmia intervalsto detect the ventricular tachyarrhythmia is reached.
 12. The method ofclaim 11, further comprising determining that the ventricular eventinterval that is greater than the pause threshold interval occurs beforethe threshold number of tachyarrhythmia intervals is reached.
 13. Themethod of claim 11, further comprising: determining that the thresholdnumber of tachyarrhythmia intervals is reached by comparing each of theplurality of ventricular event intervals to a tachyarrhythmia detectioninterval; and setting the pause threshold interval to be at least equalto or greater than the tachyarrhythmia detection interval.
 14. Themethod of claim 11, wherein determining that the rule is met comprises:determining from one of the plurality of cardiac electrical signals: afirst feature of a starting ventricular event of the ventricular eventinterval that is greater than the pause threshold interval; a secondfeature of a trailing ventricular event of the ventricular eventinterval that is greater than the pause threshold interval; determiningthat the first feature and the second feature meet pause detectioncriteria; and determining that the rule is met in response todetermining that the pause detection criteria are met.
 15. The method ofclaim 14, further comprising: determining the first feature bydetermining a first peak amplitude ; determining the second feature bydetermining a second peak amplitude; determining that the differencebetween the first peak amplitude and the second peak amplitude is lessthan a difference threshold; and determining that the first feature andthe second feature meet the pause detection criteria in response to thedifference being less than the difference threshold.
 16. The method ofclaim 14, further comprising: determining the first feature bydetermining a first morphology matching score between the startingventricular event and a morphology template; determining the secondfeature by determining a second morphology matching score between thetrailing ventricular event and the morphology template; determining thatthe first morphology match score and the second morphology match scoreare each greater than or equal to a match score threshold; anddetermining that the pause detection criteria are met in response todetermining that each of the first morphology match score and the secondmorphology match score are greater than or equal to the match scorethreshold.
 17. The method of claim 11, further comprising: determinethat at least a predetermined number of the threshold number oftachyarrhythmia intervals are less than or equal to a ventricularfibrillation detection interval; and determine that the rule is met inresponse to determining the ventricular event interval that is greaterthan the pause threshold interval and at least the predetermined numberof the threshold number of tachyarrhythmia intervals being less than orequal to the ventricular fibrillation detection interval.
 18. The methodof claim 11, further comprising: determining a median interval of apredetermined number of the plurality of ventricular event intervals;determining that the median interval is greater than supraventriculartachyarrhythmia limit; and determining that the rule is met in responseto determining the ventricular event interval that is greater than thepause threshold interval and the median interval being greater thansupraventricular tachyarrhythmia limit.
 19. The method of claim 11,wherein determining that the rule for withholding detecting aventricular tachyarrhythmia is met comprises: determining that at leasta first predetermined number of the threshold number of tachyarrhythmiaintervals are less than or equal to a ventricular fibrillation detectioninterval; determining that a median interval of a second predeterminednumber of the plurality of ventricular event intervals is greater than asupraventricular tachyarrhythmia limit; determining from the pluralityof cardiac electrical signals a difference between a first peakamplitude of a starting ventricular event of the ventricular eventinterval that is greater than the pause threshold interval and a secondpeak amplitude of a trailing ventricular event of the ventricular eventinterval that is greater than the pause threshold interval; determiningthat the difference between the first peak amplitude and the second peakamplitude is less than a difference threshold; determining a firstmorphology matching score between the starting ventricular event and amorphology template; determining a second morphology matching scorebetween the trailing ventricular event and the morphology template; anddetermining that the first morphology matching score and the secondmorphology matching score are each greater than or equal to a matchscore threshold.
 20. The method of claim 11, further comprising:determining from a next plurality of ventricular event intervals thatthe threshold number of tachyarrhythmia intervals to detect theventricular tachyarrhythmia is reached; determining that the rule forwithholding detecting a ventricular tachyarrhythmia is not met based onthe next plurality of ventricular event intervals; detecting ventriculartachyarrhythmia in response to determining that the threshold number oftachyarrhythmia intervals determined from the next plurality ofventricular event intervals is met and that the rule is not met; anddelivering a therapy in response to the ventricular tachyarrhythmiabeing detected.
 21. A non-transitory, computer-readable storage mediumcomprising a set of instructions which, when executed by a processor,cause the processor to: receive a plurality of cardiac electricalsignals; sense ventricular events from a first cardiac electrical signalof the plurality of cardiac electrical signals; determine a plurality ofventricular event intervals based on the sensed ventricular events;determine that a ventricular event interval of the plurality ofventricular event intervals is greater than a pause threshold interval;determine that a rule for withholding detecting a ventriculartachyarrhythmia is met at least in response to determining theventricular event interval that is greater than the pause thresholdinterval; determine from the plurality of ventricular event intervalsthat a threshold number of tachyarrhythmia intervals to detect aventricular tachyarrhythmia is reached; and in response to determiningthat the rule is met, withhold detecting the ventricular tachyarrhythmiawhen the threshold number of tachyarrhythmia intervals to detect theventricular tachyarrhythmia is reached.